The coalescence of two masses of the same liquid is a phenomenon of common occurrence to processes as diverse as a formation of raindrops, droplet combustion, separation of emulsions, and spray painting. Equally, but perhaps less often noticed, are examples wherein two liquid bodies that would normally be expected to unite (because they are either miscible or even the same liquid) do not, if only temporarily. For instance, splashing water in a container or on a lake when paddling a canoe often results in droplets that float on the surface for as much as several seconds before combining with the bulk liquid. Hot coffee dripping from a drip-style coffee maker routinely produces droplets that appear to dance for a few moments on the surface of the coffee already in the pot.
A scattering of droplets emerging from a jet in a nearly vertically upward fashion from a nozzle may be due to the rebound of the drops when they come into collision with one another, because droplets inevitably come into contact with each other owing to the different velocities they posses as they break away as a result of capillary instability. Even when the breakup is made more regular through the imposition of vibration, the lateral expansion of a continuous jet necessary to satisfy continuity is not possible with individual droplets, which, therefore, must come into contact. The effect of static electrical charge on both vertical jet breakup and collisions of horizontal jets has also been investigated, with results indicating both attractive and repulsive effects.
Recently, bioprocessing, such as DNA testing, on small semiconductor chip-level architectures has employed lab-on-a-chip (LOC) systems. LOC systems may use micro-machined channels or embedded electronics, for example, to move liquids from one position to another. One proposed method of moving liquids is to use a pressure pulse on a liquid in a channel to help it move along. Other methods of manipulating and mixing liquids include techniques that change the surface tension or that create surface-tension gradients or electric fields such as electro-osmosis and electro-phosphoresis; magnetic methods such as magnetohydrodynamics (MHD) stirring; rotational methods such as centrifugal forces; and acoustical methods such as acoustic streaming. Problems introduced with pipe or channel solutions include large pressure differences across a drop that are required to sustain volume flow rates due to the increase of frictional forces as the pipes or channels get smaller in cross-section. The increased friction slows down liquid transport, thus reducing device throughput. Also, with subsequent samples moving through channels and contacting channel walls, sample-to-sample contamination can be an issue.